Comp time, or compensatory time, is time off with pay instead of overtime pay. For instance, if an employee works 50 hours in a week, instead of paying overtime for the extra hours, the employee could get extra, paid time off the in the next few weeks.
Under the Fair Labor Standars Act, private sector employers can only give comp time to non-exempt employees.
If your non-profit is a private company, then you usually can't offer your non-exempt employee compensatory time.
If you're not sure whether your employee is exempt, this answer details the difference.
Before you decide to give your employee compensatory time, you should verify if your state permits compensatory time.
For example, California law provides that public, and some private sector employees, can receive comp time instead of overtime. However, as SHRM notes, California law on compensatory time conflicts with current federal law. You may want to seek legal counsel before deciding whether you can offer comp time to employees.